1. Field of the Invention
The present invention relates to an ultrasonic probe and ultrasonic probe device for use in, for example, ultrasonic endoscopes employed for medical purposes, a process for producing a piezoelectric element for use in such an ultrasonic probe and an ultrasonic diagnostic equipment and system using such an ultrasonic probe.
2. Discussion of Related Art
The common structure of conventional ultrasonic probes is as shown on page 186 of "Handbook of Medical Ultrasonic Equipments" edited by Electronic Industries Association of Japan (corporate juridical person) and published by Corona Publishing Co., Ltd., Tokyo, Japan, and they are fabricated by bonding a piezoelectric element composed of a piezoelectric ceramic plate as represented by PZT whose two principal surfaces are provided with a pair of electrodes to an acoustic backing layer and further bonding an acoustic matching layer and an acoustic lens thereto.
In this ultrasonic probe, a pulser applies pulsed energizing voltage of about one hundred or hundreds of volts to the above piezoelectric element, so that a rapid morphology change is caused to occur by the converse piezoelectric effect of the piezoelectric element. The resultant vibration is transmitted through the acoustic matching layer and the acoustic lens to thereby effectively transmit pulses toward an object to be observed. The transmitted pulses are reflected at each tissue interface of the body in medical uses or at a noncontinuous part such as any of various defects inside an object to be examined in nondestructive inspections and retransmitted through the acoustic lens and the acoustic matching layer to thereby apply mechanical vibration to the piezoelectric element. This mechanical vibration is converted to electrical signal by the piezoelectric effect of the piezoelectric element, which is observed by means of a diagnostic equipment.
Various methods are available for enhancing the resolution in the imaging, which include modifying the acoustic lens and the pattern of the electrode of the piezoelectric element and curving the surface of the piezoelectric element itself so as to slenderize the transmitted ultrasonic beam. Further, an annular array sound field forming is being tried in which the electrode is split into several parts to thereby cause applied energizing pulse voltages to have differences.
Moreover, with respect to the ultrasonic probe, for example, an invention is noted which is described in Japanese Patent Application Laid-Open Specification No. 111198/1990. In this invention, for example, a full covering electrode 152 is disposed on one side of a piezoelectric ceramic 151 while split electrodes 153a, 153b, 153c are disposed on the other side of the piezoelectric ceramic as shown in FIGS. 47 and 48. Thus, the invention provides a piezoelectric element 154 in which, with respect to the distribution of polarization intensity, the central split electrode 153a has a strong spontaneous polarization, the spontaneous polarization becoming weaker as the distance to the peripheral split electrode 153c is decreased, and also provides an ultrasonic probe including this piezoelectric element 154.
However, all the conventional methods of improving the resolution of an ultrasonic image of the ultrasonic probe have had respective advantages and disadvantages. In particular, the ultrasonic probe including an acoustic lens or a piezoelectric element that has a curving, especially concave, surface advantageous in that the ultrasonic beam is slenderized near the focus to thereby enable obtaining an image of high resolution but has a drawback in that the beam width is enlarged as the distance from the focus is increased to thereby deteriorate the image quality. Accordingly, the optimum observation distance permitting practical observation becomes unfavorably small.
In this connection, measuring the sound field of ultrasonic beam transmitted from the ultrasonic probe and determining the beam width thereof demonstrates that the region where the beam width is small is extremely reduced so that the optimum observation distance (depth of focus) permitting practical observation is small as shown in FIG. 49 (in FIG. 49, the axis of abscissa represents the distance X (mm) between the face transmitting ultrasonic beam and the measuring point while the axis of ordinate represents the beam width (mm) at the measuring point; the above beam width means the portion of at least 50% (-6 dB) of the maximum acoustic pressure in a plane at right angles to the beam axis, the beam width meaning the diameter of the area thereof).
In the use of the ultrasonic probe of the annular array type, it is known that the focusing point of ultrasonic beam can be changed (the depth of focus can be increased) by, for example, providing a plurality of pulsers or devising the circuit to thereby control the phase of voltage applied to each ring-shaped electrode.
That is, the ultrasonic wave can be converged so as to focus on a point to be observed to thereby enable clear observation of the point. Further, it is feasible to repeat ultrasonic wave transmission and receiving a plurality of times while changing the focal length and synthesize received signals so that a clear image is obtained in a wide range.
However, in the above method, a plurality of leads must be connected to the piezoelectric element. Complete shielding is required for avoiding the entry of noise between the plurality of leads. This is disadvantageous from the viewpoint of the reduction of the diameter of the ultrasonic probe for use in medical applications and the like. Further, there is the problem that the requirement for a complex electrical circuit capable of phase control as mentioned above would cause a huge increase of the cost of the diagnostic equipment.
In the method comprising repeating transmission and receiving while changing the focal length and synthesizing a plurality of received signals, the time required for gaining a sheet of image is increased, so that the frame rate (number of images per time) is lowered. Consequently, the problem occurs that the image is fluctuated by, for example, the somatic movement, respiration or heart beat of the examinee to thereby disenable satisfactory diagnosis.
In the use of a concave transducer as another usage of the annular array type, a technique is available such that all parts are simultaneously driven at the time of transmission to thereby transmit ultrasonic wave while the area of receiving parts is increased in accordance with the observation distance at the time of receiving to thereby attain enhanced resolution. However, as in the above method, this technique encounters problems such that not only is the wiring complex to thereby render production difficult but also the electrical circuit such as an adder circuit becomes complex so that high cost and lowered reliability would result.
Distributing the ultrasonic wave transmitted by an ultrasonic transducer so as to be strong in the center and weak in the periphery enables reducing the side lobe, i.e., transmission of ultrasonic wave in essentially unintended directions. It is known that the side lobe reducing effect is especially high when a provided acoustic pressure distribution has a graphical profile of the normal distribution (Gaussian distribution). The object of the invention of Japanese Patent Application Laid-Open Specification No. 111198/1990 is directed toward this effect, but the invention has drawbacks in that there is no effect of focusing ultrasonic wave with the optimum observation distance being small.
It is known that distributing the ultrasonic wave transmitted by an ultrasonic transducer in a profile of the zeroth-order Bessel function would cause the ultrasonic wave to become diffractionless ultrasonic beam, i.e., wave being propagated without diffusion. The use of the thus obtained diffractionless ultrasonic beam enables obtaining image produced by ultrasonic beam of uniform width from near the ultrasonic transducer to a position distant therefrom. That is, image of line focus can be obtained realizing clear focusing from near the ultrasonic transducer to a position distant therefrom.
The method was reported for realizing the above ultrasonic beam, in which use is made of a piezoelectric element provided with concentric multi-ring electrodes to each of which a driving circuit is connected by means of cables and in which the energizing pulse voltage applied to each electrode ring is distributed in a Bessel profile. However, in this method, a plurality of cables must be connected to the piezoelectric element. Complete shielding is required for avoiding the entry of noise between the plurality of cables. This is disadvantageous from the viewpoint of the reduction of the diameter of the ultrasonic probe for use in medical applications, especially, applications inside the body cavity. Further, there is the problem that the requirement for a complex electrical circuit capable of effecting voltage and phase controls would cause a grave increase of the cost of the diagnostic equipment.
The following reference describes a method of fabricating a piezoelectric element of the Bessel polarization type which has similarity to the piezoelectric element for use in the present invention:
title: Bessel beam ultrasonic transducer: Fabrication method and experimental results, PA1 authors: D. K. Hsu, F. J. Margetan and D. O. Thompson, and PA1 publication: Appl. Phys. Lett. 55(20), 13 Nov. 1989, pp 2066-2068. PA1 a piezoelectric element is fabricated which is provided with two concentric circular grooves of different depths and a central cavity, PA1 polarization is effected by the application of polarization voltages having polarities different between the grooves and the face opposite thereto; at that time, mutually neighboring grooves and cavity have respective voltages of different polarities applied thereto, and PA1 the grooves are polished to thereby remove the same, so that a flat piezoelectric element is obtained. PA1 polarization voltage is applied not only between the electrodes opposite to each other with the piezoelectric element interposed therebetween but also between the electrodes neighboring to each other, so that the occurrence of part 155 polarized parallel to a principal surface of the piezoelectric element 154 as shown in FIG. 50 is inevitable with the result that undesired vibration is likely to occur at the time of driving with voltage applied to thereby transmit ultrasonic wave of undesired frequency in undesired directions, which generates virtual image attributed to artifacts on the ultrasonic image to thereby deteriorate the quality of the ultrasonic image, and PA1 major portion of the piezoelectric element is polished away after the polarization, so that the fabrication time is prolonged and that the yield becomes poor because of the required grooving and polishing after the polarization.
The above reference employs the following method in which:
However, the piezoelectric element obtained by the method of this reference has drawbacks in that: